Known techniques for scanning electron microscopes includes U.S. Pat. No. 5,250,808 invented by Danilatos et al. entitled "Integrated Electron Optical/Differential Pumping/Imaging Signal System For An Environmental Scanning Electron Microscope" that is owned by Electroscan Inc. is hereby incorporated by reference. This reference teaches of a gaseous environmental electron microscope (ESEM) in which the present invention makes improvements upon which can also be used in other electron microscopy devices. In particular, the improvements are of the dual aperture carrier and secondary electron imaging detector assembly as shown in FIG. 1 herein which is also disclosed by this reference. Held within the assembly are two apertures for differential pumping. When a bottom aperture is greater than 2.0 mm, vacuum within the ESEM chamber degrades dramatically. Smaller bottom apertures limit the degree of allowable lower magnification. The improvement presented herein allows magnification below 20X without vacuum failure. Another deficiency with this prior art ESEM apparatus includes the imaging detection hardware that is used for detecting secondary electrons that strike the back of the detector. This detector design induces extraneous noise in the detected signal of the examined specimen.
Another prior art references that encompass the present invention includes U.S. Pat. No. 4,785,182 invented by Mancuso et al. entitled "Secondary Electron Detector For Use in A Gaseous Atmosphere" which is also incorporated by reference. This reference teaches the conceptual method and means to implement U.S. Pat. No. 5,250,808 discussed above. However, this teaching does not discuss a method or means that allows for lower magnification capabilities in ESEM apparatus.
Another teachings that bears upon the U.S. Pat. No. 4,785,182 and is discussed that patent and appears to limit the type of allowable detection schemes for ESEH apparatus is the article by Danilatos entitled "A Gaseous Detector Device for the Environmental SEH" in Micron and Microscopa Acta, Vol. 14(4), (1983), pp. 307-318. This article states ". . . it was not possible to obtain secondary electron image for a direct comparison. Alternatively some sort of trajectory contrast might be present." This statement infers that the images derived from ESEH apparatus were secondary electrons (SE), backscatter electrons (BSE), a combination of both SE and BSE, absorbed current, or yet another method of signal detection. In the U.S. Pat. No. 4,785,182, its' disclosure restricts the means of electron detection by the detector to only secondary electrons. However, since that teaching discloses only detectors with direct line-of-sight with the examined specimen, one can infer that other types of electrons such as BSE are being detected as well. Thus, the present invention allows for other types of detector schemes in a ESEH type SEM.